1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the reticle and the substrate is disposed a projection system to image the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components to direct, shape and/or control the beam of radiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
A consideration in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features, and one of these is the wavelength of the radiation used to expose the pattern.
It is expected that the use of EUV lithography will enable the manufacture of feature sizes below 32 nm using radiation with an EUV (extreme ultra violet) wavelength between 5 and 20 nm, and typically 13.5 nm. Radiation at this wavelength is absorbed in most materials, and conventional refractive optics are generally considered to be unsuitable for use with such radiation. The optics in a projection system for use with EUV lithography is therefore based on mirrors, which operate in a high vacuum environment. The projection system is therefore enclosed in a projection optics box (POB) which is kept under vacuum.
Similar considerations apply for lithography using radiation having a wavelength falling outside the EUV band. For example, a projection system for lithography using radiation having a wavelength of 193 nm may also include mirrors instead of, or in addition to, refractive optics. The projection system may therefore need to be kept under a vacuum or at least in a controlled environment for non-EUV lithography.
Furthermore, considerations that apply to the projection system will also apply to the illumination system used to supply the beam of radiation to the reticle. As with the projection system, the illumination system includes components to direct, shape and/or control the beam of radiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example. As with the projection system, the illumination system may need to be kept in a controlled environment or under vacuum.
The projection system and/or illumination system generally also includes elements to set the numerical aperture NA of the projection system and/or illumination system. In some prior art systems an aperture adjustable NA-diaphragm may be provided in a pupil of the projection system and/or illumination system. Particularly in the case of EUV lithography, the space around the optical components is very restricted, making the use of an adjustable diaphragm impractical.
In addition, it may be desirable to use apertures having a shape other than a circle. For example, an elliptical aperture is useful for off-axis reflective systems. It is practical to produce apertures having such shapes using an adjustable diaphragm.
A proposed solution to the unsuitability of adjustable diaphragms is the provision of a series of individual stop discs, each having an opening with a fixed aperture, to adjust the NA stop. The stop discs are loaded into a mechanical disc changer mechanism which can place one of the discs at a time in the pupil of the projection system. A single stop disc occupies considerably less space than an adjustable diaphragm, and enables the shape of the NA stop to be determined more accurately.
Optics which operate under high vacuum should avoid contamination. Contaminants (e.g. particles, molecules) can lead to a deterioration in the reflectivity of the mirrors. Due to the exceptionally small feature size, particles as small as 50 nm can lead to failures on the finished substrate. The introduction of a magazine of interchangeable stop discs and a mechanical disc changer mechanism into the projection system and/or illumination system increases the chance of introducing contaminants into the projection system and/or illumination system at the same time.
The complex mechanical disc changer must also be reliable. The projection system is generally situated in the very center of a lithographic apparatus, and any repair involves breaking the vacuum and completely disassembling the apparatus. This process is time consuming and expensive, and may also lead to contamination within the projection system. The disc changer should be designed to be as reliable as practicable, but nevertheless may need to be serviced on occasion. Furthermore, in some circumstances it might be desirable to modify the discs available to the disc changer, for example to modify the range of NA stops available to the lithographic apparatus. This is again impossible without breaking the vacuum and disassembling the apparatus.